This invention relates to a core catcher, a manufacturing method of a core catcher, a reactor containment vessel and a manufacturing method of a reactor containment vessel.
In a water cooled reactor, by rundown of water supply into a reactor pressure vessel or a rupture of piping connected to the reactor pressure vessel, a reactor water level may fall, a reactor core may be exposed above the water level and cooling may become insufficient. Supposing such a case, it is designed that a nuclear reactor is shut down automatically under a signal of low water level, the reactor core is covered and cooled by water injected by an emergency core cooling system (ECCS), and a core meltdown accident is prevented.
However, although it is a very low probability, it can be assumed that the above mentioned emergency core cooling system would not operate and any other devices for supplying water to the reactor core would not be available. Under such a condition, the reactor core would be exposed due to lowering of the reactor water level and cooling would be insufficient, fuel rod temperature would rise with decay heat generated continuously after shutdown of the nuclear reactor and the reactor core would meltdown eventually.
If such a severe accident occurs in the nuclear power plant, the molten core would penetrate the reactor pressure vessel lower head at bottom of the reactor pressure vessel and would fall to a floor of reactor containment vessel. Core debris, a wreckage of the molten core, continues to generate heat as about 1% of reactor thermal power because of decay heat of radioactive material that exists inside. Therefore, if there is no means for cooling, the core debris heats concrete stretched on the containment vessel floor. If temperature of contact surface is high, the core debris would react with the concrete and generate large quantity of non-condensable gas, such as carbon dioxide or hydrogen, while melting and eroding the concrete. Eventually, a lot of radioactive material would be emitted to the environment.
The generated non-condensable gas would pressurize and damage the reactor containment vessel and would damage a containment vessel boundary by melting erosion of concrete or reduce structure toughness of the containment vessel. As a result, if the reaction of the core debris and the concrete continues, it would result in a breakage of the containment vessel and a radioactive material in the containment vessel would be emitted to the outside.
In order to suppress such a reaction of core debris and concrete, it is necessary to cool the core debris so that temperature of the surface of the concrete contacting with a bottom of the core debris is below erosion temperature (1500K or less for typical concrete) or to avoid that the core debris contact directly with the concrete. In a conventional way, it is designed to suppress the reaction of melting and eroding the concrete by pouring water over the fallen core debris and lowering temperature of the core debris (for example, refer to Japanese Patent Application Publication 2004-333357 and Japanese Patent Application Publication 2005-195595; the entire content of which is incorporated herein by reference).
Various countermeasures are proposed against falling of the core debris. A typical one is a core catcher. The core catcher catches and holds the fallen core debris on heat resistant material and cools the core debris with means for supplying water.
The core catcher is a safety equipment that assures soundness of the reactor containment vessel by catching the core debris and maintaining it cooled and reduces emission of radioactive material to the outside.
In the existing boiling water nuclear power plants (BWR), the probability of occurrence of an accident is suppressed. And very high safety relating to core cooling during an accident is achieved. Such a severe accident has never occurred. Also in a probability risk analysis (PSA), the probability of occurrence of such a severe accident is evaluated so small as it can be ignored.
Today, a natural circulation cooling type passive safety boiling water reactor (ESBWR) which constitutes all safety systems with static instruments is proposed. In the ESBWR, the core catcher is installed beneath the reactor containment vessel. This is for further improving completeness of the safety of next generation BWR.
If a corium is cooled by boiling of water supplied over the corium at a top surface and a deposition thickness of the corium is so thick, it may not be able to cool the corium fully to the bottom of it. Therefore, it is necessary to make floor area large and to reduce the deposition thickness of the corium so that it can be cooled. However, a structural design of the containment vessel makes it difficult to expand the floor area sufficiently.
For example, typical decay heat of corium is about 1% of rated thermal power. In case of a power reactor of 4,000 MW of rated thermal power, the decay heat is about 40 MW. Although an amount of boiling heat transfer on top surface varies depending on the condition of the top surface of the corium, heat flux of about 0.4 MW/m2 can be assumed as the smallest value. In this case, supposing that heat of the corium is removed only by heat transfer at the top surface, about 100 m2 (11.3 m of a diameter) of floor area is necessary. Therefore, as a thermal power of a plant becomes large, necessary floor area of lower drywell becomes large and it becomes more difficult to design the containment vessel.
In case that cooling water is supplied over the top surface of the core debris fallen to the floor of the reactor containment vessel, if an amount of removable heat at the bottom of the core debris is small, temperature at the bottom of the core debris may remain high because of decay heat and erosion of concrete of the containment vessel floor may be unable to be stopped. Therefore, some methods for cooling from bottom of the core debris are also proposed (for example, refer to Japanese Patent Application Publication 2005-195595, Japanese Patent Application Publication Hei 7-110392, Japanese Patent Application Publication Hei 6-130169 and Japanese Patent Application Publication Hei 9-138292; the entire contents of which are incorporated herein by reference).
The core catcher is located on the floor of the lower drywell with heat resistant member for example so that the core debris does not penetrate the lower part of the reactor containment vessel or radioactive material does not leak. However, the core debris might not be cooled sufficiently only by covering with heat resistant member. And it takes long time and labor to provide a lot of piping for cooling water to run in order to cool the core debris.
If the cooling water is supplied only over the corium, the corium is cooled only by boiling of the water at the top surface of the corium. So, if deposition thickness of the corium is so large, it may be unable to cool sufficiently to the bottom of the corium. Therefore, large floor area is necessary to make the corium as thin as it can be cooled. However, structural design of the containment vessel makes it difficult to provide large enough floor area.